Polylactic acid is an eco-friendly biodegradable plastic, which is produced with the use of lactic acid as a raw material. To improve the quality of polylactic acid, L-isomer or D-isomer of lactic acid with high optical purity is required. In this regard, lactic acid obtained by fermentation is more advantageous than lactic acid synthesized from petroleum, because of its high optical purity.
Depletion of oil resources is predicted to occur in the future. Substitution of petroleum-derived plastic with polylactic acid will be required. However, polylactic acid is more expensive than petroleum-derived plastic, and thus further reduction in lactic fermentation cost is required.
In general, lactic fermentation is performed using food plant biomass such as corn (maize) as a carbon source, and thus lactic acid production with the (easy) use of a food resource as a raw material causes rising food prices. Accordingly, development of a lactic fermentation technique with the use of nonfood plant biomass that does not compete with foods is also desired.
Typical known examples of nonfood plant biomass include non-edible parts of rice and oats, such as rice straw and wheat straw. Rice straw is disposed by open burning or being plowed into agricultural fields, but disposal differs depending on area. Rice straw is an unused resource, with only about a half thereof is used. However, cellulase for glycosylation of rice straw is expensive, and thus rice straw is not easily used.
Hydrolysis using concentrated sulfuric acid is employed for glycosylation of woody biomass. However, this method is problematic in terms of neutralization cost after treatment with concentrated sulfuric acid, also because it increases environmental burdens due to the use of concentrated sulfuric acid. Furthermore, the method requires purification of glucose from a solution treated with concentrated sulfuric acid, causing a further increase in cost.
Known lactic fermentation involves performing 20 minutes of sterilization at 120° C. for medium before inoculation of lactic acid bacteria. This sterilization not only requires expensive equipment such as a boiler for a culture apparatus, but also results in a large cost for sterilization. Furthermore, pressure is applied for sterilization, and thus a firm container is required, which leads to high initial cost.
Meanwhile, other known lactic fermentation techniques using plant biomass as a raw material are implemented with an open system that involves no sterilization (patent documents 1 and 2). With a technique that involves inoculation of large amounts of lactic acid bacteria (patent document 1), about 2.5% L-lactic acid can be obtained per culture solution within 3 days. Also, with a lactic fermentation technique (patent document 2) implemented at a high temperature of about 45° C., about 4% L-lactic acid can be obtained per culture solution within 2 days.
However, a method that involves inoculation of large amounts of lactic acid bacteria requires culturing large amounts of cells upon lactic fermentation, resulting in much cost and labor. However, lactic fermentation at a high temperature (about 45° C.) requires expenditures for maintaining a fermentation tank at such temperature. Furthermore, these lactic fermentation techniques have drawbacks in that only about up to 4% lactic acid is obtained per culture solution, resulting in a high cost for lactic acid purification following fermentation.
Therefore, a method required herein is to perform lactic fermentation from plant biomass at room temperature with small amounts of bacteria to be inoculated and without performing sterilization.